In the experimental work, which requires lots of efforts to get reliable data, LDA (Laser Doppler Anemometry) and PIV (Particle Image Velocimetry) are generally used to measure the velocity profile and turbulent intensity in the sub-channels. For the purpose to improve the thermal hydraulic performance of the fuel assemblies, many experiments have been conducted to understand the flow structures and three-dimensional more » behaviors of the turbulence in the rod bundle sub-channels and then to optimize the design of the spacer grid. Heat transfer between fuel rods and coolant is a crucial part of thermal-hydraulic analysis. Hot fuel rods are cooled by the coolant water flow, and the spacer grids with mixing vanes distort the mainstream, producing the swirling flow which can enhance the heat transfer to improve CHF (Critical Heat Flux). Pressurized water nuclear reactor fuel assemblies are made of fuel rods held by spacer grids, which are the core components of the nuclear power plants. The effectiveness of the method is emphasized by the simplicity of the result and by the conformity with published data. Inversion of this result then allows the particle eddy viscosity to be determined in terms of existing empirical or quasi-empirical formulae for the minimum suspension velocity. The basis of the calculation procedure is that the advection/diffusion method is used to determine the minimum suspension velocity in terms of the particle eddy diffusivity. The objective of this article is to develop a new and concise expression for the particle eddy diffusivity in turbulent pipe flow.
Theoretical models of the process describe the process variously in terms of: (1) the turbulent energy required to support the particles (2) a force on the particles due to the turbulent eddies which balances the force of gravity and (3) a balance between a downward drift (advection) caused by gravity and diffusion by turbulence against the induced particle more » concentration distribution. It is generally agreed that at a sufficiently high flow rate, particles remain in suspension because the effect of the turbulent eddies is sufficiently strong to overcome the tendency of the particles to fall under gravity to the bottom of the pipe.